Battery saving techniques have become increasingly important in modern messaging systems that employ battery powered portable subscriber units. In general, such systems have transmitted messages in a plurality of cyclical time slots. Each subscriber unit has been assigned to a predetermined subset of the time slots and thus can remain in a low-power, "battery saving" mode during all time slots except the predetermined subset to which the subscriber unit is assigned.
Motorola's well-known FLEX.TM. protocol is a good example. The FLEX.TM. protocol utilizes a repeating transmission cycle consisting of 128 frames every four minutes. Each subscriber unit is pre-programmed by a "pager collapse" value between zero and seven to monitor every frame, every other frame, every fourth frame, every eighth frame, every sixteenth frame, every 32nd frame, every 64th frame, or every 128th frame, respectively. For best battery life, the subscriber unit is pre-programmed to monitor only every 128th frame (the longest battery saving interval). For fastest delivery of messages (lowest delivery latency), the subscriber unit is pre-programmed to monitor every frame. Thus, battery life and delivery latency can be traded off, depending on the pager collapse value chosen. In addition, the FLEX.TM. protocol accommodates a "system collapse" value transmitted at the start of each frame. The system collapse value affects all subscriber units in the system. In the event that the system collapse value and the pager collapse value are different in a given subscriber unit, the value which causes the subscriber unit to monitor the greater number of frames per cycle applies.
While the pager collapse value and the system collapse value have worked reasonably well for providing a pre-programmed trade-off between battery life and delivery latency and for overcoming certain queuing imbalances, neither provides a dynamically adjustable battery saving interval that can be custom tailored for an individual subscriber unit. A custom tailored, dynamically adjustable battery saving interval is needed, because events can occur in the messaging system that can temporarily alter the optimum trade-off between battery life and delivery latency for a given subscriber unit.